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Generate Your Scientific Gas Supply On-Site ... Gas Generators - The Alternative to Cylinder Gas

The quality of carrier, detector and purge gasses is crucial in obtaining optimum results from analytical instruments. A reliable supply of consistently pure gas is the basis of any analytical laboratory but is commonly overlooked. This paper examines the increasing use of gas generators as a superior alternative to a cylinder gas supply.

Firstly we shall examine what sorts of gases can be generated at the point of use and the technologies involved in producing them, and we will then investigate the potential benefits that point of use generation provides over cylinder gas supplies.

Within most analytical laboratories there are four major gases required. Hydrogen as a fuel for flame ionisation detectors and as a carrier gas for GC's, nitrogen as either a make-up gas or an oxygen free purge gas, air with CO2 removed from it as a purge gas for instruments utilising infrared light detection techniques (FT-IR) and hydrocarbon free air for use as a combustion gas in both GC & TOC instruments.

Hydrogen Generation

Hydrogen Generators work on the basic principle of the electrolysis of water.

Water is fed into an electrolytic cell where it is then split into hydrogen and oxygen. The oxygen is vented to the atmosphere and the hydrogen is available for use in the laboratory. The basic principles of hydrogen generation have been around for some time; however, it is only recently that technologies have advanced sufficiently to allow for very high purity separation of the hydrogen and oxygen, giving the very low levels of oxygen contamination (typically <1 ppm) within the hydrogen stream. The purity of the outlet hydrogen is ensured by passing it through a filtration column, which ensures that the gas has a pressure dewpoint of -70oC. Water supply to the generator is automatically controlled by an internal level sensor and an automatically controlled feed pump,

Nitrogen Generation

Nitrogen generation is a comparatively new concept (compared to Hydrogen Generation) as the technologies involved allowing nitrogen to be separated from atmospheric air have become available. The principle of operation (as shown in the following figure) is based on a pressure swing adsorption system and relies on the varying molecular sizes of the constituent components of the atmospheric air (nitrogen being one of the largest).

Compressed air is fed into a bed of carbon sieve material, which will differentially adsorb oxygen, carbon dioxide and hydrocarbons allowing pure nitrogen to pass out at the top of this sieve bed and into a holding internal reservoir. In order to regenerate the sieve bed and purge it of impurities, it is periodically vented to atmosphere on a carefully controlled time cycle. This sudden change in pressure down to atmosphere is sufficient to allow total desorbtion of all the which had, defuses into the sieve material. This cycling process is transparent to the end user of the produced nitrogen as the internal holding reservoir has sufficient capacity to provide a continuous flow of nitrogen even during the periods when the carbon sieve bed is being regenerated. The quality of gas seen at the output of the generator has very low levels of contamination (typically < 5 ppm oxygen) and like the raw material (atmospheric air) is available in an endless supply.

CO2

CO2 removal from compressed atmospheric air can be achieved in a similar way to nitrogen. The process again involves pressure swing adsorption, however the molecular sieve involved is activated alumina, which will preferentially bond to carbon dioxide due to the polar effects of this molecule. The principles of operation are shown in the following figure.

It can be seen that compressed inlet atmospheric air is passed through a prefilter and then is directed by means of a control valve up one of two sieve tubes. The air, which passes out of the top, is free from moisture and carbon dioxide. This air then passes through a final filtration process and is available to the laboratory. Part of the output air is redirected back down through the unused sieve tube to act as a purifying air stream purging the trapped impurities out the bottom of the tube to atmosphere. Every two minutes the tubes are alternated by means of a timer mechanism. This means that the sieve tubes can never become completely saturated with impurities before they are desorbed and regenerated. The output gas stream typically has <1 ppm of carbon dioxide present and also achieves a pressure dewpoint of -70oC.

Zero Air Generation

The quality of air used as a combustion gas for Gas Chromatographs has a direct bearing on the levels of noise seen in the base line of the detector and consequently the dynamic range of the GC. In order to remove any hydrocarbons present in the inlet air, which would cause noise on the FID baseline, we have to catalytically oxidise these impurities by means of a heated platinum catalyst as seen in the figure following.

The inlet compressed air is passed through a prefilter and then into a heated chamber which contains platinum coated beads. This chamber is maintained at a temperature of 450oC, which is sufficient to supply the required energy to initiate the dissociation of the carbon-hydrogen bonds of any impurities in the inlet air. The hydrocarbon free air then passes through a cooling coil and a final filtration process before it is available as a pure combustion gas. This unit can achieve levels of <0.1 ppm total hydrocarbon concentration in the outlet air supply.

The Benefits

As an example of the benefits of gas generation, three separate illustrations will be used.

1. Mobile Laboratories.
Many mobile laboratories operate at least one GC that necessitates the hydrogen, nitrogen and air supplies all readily available
  • The weight of each gas cylinder
  • The impact of the cylinder weight in the truck regarding modifications to the suspension and reinforcements to the chassis
  • Safety regulations regarding transportation of high pressure cylinders
  • Back up cylinder to ensure a continuous supply of each gas is on hand
The sum total of the three gas generators (hydrogen generator, nitrogen generator and zero air generator) weigh less than 1 cylinder, the gas generators provide endless and consistent pure gas pressures with the generators being <150 psi safety was not an issue. Based on consideration of the above issues, mobile laboratories .

2. Safety and Above Ground Level Laboratory

Safety is always an issue and can be a major concern when laboratories are sited above ground level in a building. Occupational Health & Safety regulations plus individual Company safety guidelines all must be considered as well as the logistics associated with the safe supply of gas. A research laboratory in Wilton in the United Kingdom faced these problems. The laboratory was sited on the third floor of an office block in a town centre. Changing regulations dictated that the location of a cylinder within the office block was prohibited. Whereas before the laboratory had logistical problems of bringing a heavy high pressure cylinder to the third floor of the building, they were now faced with an expensive bill for constructing an external cylinder store and gas piping to their laboratory. A small bench top hydrogen generator, nitrogen generator and zero air generator provided the solution.

In Australia, pharmaceutical companies often have to conform to parent company safety guidelines. One example is Hydrogen cylinders can be prohibited anywhere on their site and gas generators are mandatory for the production and supply of hydrogen gas.

Other areas where gas generators as alternatives to cylinder supply should be considered and reviewed are:

  • Minimising laboratory plumbing
  • Planning and designing new or renovated plumbing
  • Expanding gas consumptions and new instrumentation
  • Repair of reticulation system, leak detection in pipework, and pressure gauge repairs or replacement.

Gas Generators offer:

  • Continuous supply
  • Consistent high purity
  • Total safety
  • Cost savings - economies of scale
  • Space saving
  • Time saving
  • Easy and quick relocation
On Site Generating Solutions Pty Ltd is the exclusive Australian distributor of the Noblegen range of High Purity Gas Generators. A range of technical articles covering gas selection for GC applications is also available.

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